(a) Field of the Invention
The present invention relates to a semiconductor light-emitting device having a light-emitting diode or the like, and particularly to a semiconductor light-emitting device used for lighting modules or lighting apparatuses, and a method for fabricating the same.
(b) Description of Related Art
Recently, as the luminance of a white light-emitting diode (hereinafter referred to as “LED”) increases, the application of LEDs is greatly expanded from for indication to for illumination.
In general, the white LED has a structure in which a semiconductor light-emitting element for emitting blue or ultraviolet light in combined with a phosphor that is excited by the blue or ultraviolet light emitted from the semiconductor light-emitting element. The semiconductor light-emitting element includes a semiconductor layer comprising a p-type electrode and an n-type electrode, and the semiconductor layer has a pn junction structure formed by depositing a gallium nitride compound semiconductor layer on a sapphire substrate.
However, since the sapphire substrate has a electrical insulating property, there is a need to form an n-type electrode and a p-type electrode on a surface of the gallium nitride compound semiconductor layer that is opposite to the surface on which the sapphire substrate is formed. In semiconductor light-emitting devices employing such semiconductor light-emitting element, the following feeding method is generally used.
In a semiconductor light-emitting device using a first feeding method, a heat sink is bonded via Ag paste or the like to the surface of the sapphire substrate that is opposite to the surface on which the gallium nitride compound semiconductor layer is formed. The n-type and p-type electrodes, which are formed on the upper surface of the gallium nitride compound semiconductor layer of the semiconductor light-emitting element, are electrically connected to a feeding unit through a bonding wire. Therefore, in the semiconductor light-emitting device using the first feeding method, the n-type and p-type electrodes are fed through the bonding wire.
In a semiconductor light-emitting device using a second feeding method, a p-type electrode and an n-type electrode are formed at the upper level of the gallium nitride compound semiconductor layer deposited on the sapphire substrate. Moreover, patterned electrodes are formed on a heat sink and the patterned electrodes are flip-chip mounted using Au bumps through which the patterned electrodes are electrically connected to the p-type and n-type electrodes. Therefore, in the semiconductor light-emitting device using the second feeding method, the n-type and p-type electrodes are fed through the patterned electrodes formed on the heat sink.
The semiconductor light-emitting device using the first feeding method has the sapphire substrate interposed between the semiconductor light-emitting element and the heat sink. The sapphire substrate has large thermal resistance and thus poor heat dispersion. Therefore, the semiconductor light-emitting device using the first feeding method cannot achieve sufficient heat dispersion. As a result, when the semiconductor light-emitting device is applied to illumination or the like that requires a large mount of power, problems such as thermal saturation of light output and decrease of reliability of light output arise.
In addition, in the semiconductor light-emitting device using the second feeding method, only Au bumps serve as paths for heat dispersion and, therefore, sufficient heat dispersion cannot be achieved. As a result, also when the semiconductor light-emitting device is applied to illumination or the like that requires a large amount of power, problems such as thermal saturation of light output and decrease of reliability of light output arise.
To solve the problems, the following semiconductor light-emitting device has been proposed (see, e.g., D. Morita et al., “High Output Power 365 nm Ultraviolet Light Emitting Diode of GaN-Free Structure,” Jpn. J. Appl. Phys. Vol. 41 (2002), pp. L1434-L1436).
FIG. 15 shows a cross sectional view of a known semiconductor light-emitting device.
As shown in FIG. 15, a p-side ohmic electrode 802, a pn junction structure 803, and an n-side ohmic electrode 804 are formed on a support 800, which is a heat sink and is made of CuW, with a fusion material 801 interposed between the support 800 and the p-side ohmic electrode 802. The pn junction structure 803 is a semiconductor layer which is formed by sequentially depositing a p-type gallium nitride compound semiconductor layer, an active layer, and an n-type gallium nitride compound semiconductor layer.
In this manner, over the support 800, a semiconductor light-emitting element is formed which includes the semiconductor layer (pn junction structure 803), the p-side ohmic electrode 802 underlying the semiconductor layer, and the n-side ohmic electrode 804 overlying the semiconductor layer.
On the n-side ohmic electrode 804, an Au plating layer 805 is formed which is electrically connected to a power supply pole 807 through an Au wire 806.
Now, how to complete the pn junction structure 803 will be described in detail.
First, a pn junction structure 803 on which a semiconductor layer is epitaxially grown is formed on a sapphire substrate (not shown). Then, the pn junction structure 803 formed on the sapphire substrate is peeled off therefrom using a laser lift-off technique, and the pn junction structure 803 is bonded via a fusion material 801 on a support 800. In this manner, the pn junction structure 803 formed on the sapphire substrate is peeled off therefrom by laser lift-off technique and then bonded on the support 800.
The conventional semiconductor light-emitting device allows the semiconductor light-emitting element to emit light by feeding a current to the power supply pole 807 and the conductive support 800 from outside.
In the conventional semiconductor light-emitting device, the semiconductor light-emitting element is formed via the fusion material 801 on the support 800 which is a heat sink, not on the sapphire substrate.
With this structure, the entire principal surface of the semiconductor light-emitting element is bonded via the fusion material 801 to the support 800 to achieve good heat dispersion in the semiconductor light-emitting device.
Moreover, in the conventional semiconductor light-emitting device, the semiconductor light-emitting element including the semiconductor layer (pn junction structure 803), the p-side ohmic electrode 802 underlying the semiconductor layer and the n-side ohmic electrode 804 overlying the semiconductor layer is formed over the support 800.
With this structure, the conventional semiconductor light-emitting device can reduce series resistance as compared with a semiconductor light-emitting device using a semiconductor light-emitting element which includes a semiconductor layer (pn junction structure) and p-side and n-side ohmic electrodes on the semiconductor layer. As a result, the conventional semiconductor light-emitting device can suppress heat generation therein.
However, the results of the inventors' experiments on the conventional semiconductor light-emitting device using laser lift-off technique have shown the following problems.
The pn junction structure 803 peeled off from the sapphire substrate (not shown) by laser lift-off technique is as extremely thin as several μm to over 10 μm. Therefore, during wire bonding, the semiconductor light-emitting element is deformed by application of ultrasound or local application of pressure from the tip part of a collet for wire bonding, and thereby causes cracking or chipping. As a result, there found a problem of significantly reducing yield of the semiconductor light-emitting device.
Moreover, the semiconductor light-emitting element is formed over the support 800 made of a material having a hardness lower than that of sapphire, SiC or the like. Therefore, during the wire bonding, the support 800 is easily deformed by loads placed through the application of ultrasound or local application of pressure from the tip part of the collet for wire bonding to the semiconductor light-emitting element, and thereby causes a deformation of the semiconductor light emitting element. As a result, there found a problem of generating cracking or chipping in the semiconductor light-emitting element.
Furthermore, in the case of a conventional semiconductor light-emitting device in which a plurality of semiconductor light-emitting elements are arranged in an array, power is supplied to each of the semiconductor light-emitting elements through bonding wires. Therefore, there found a problem that the larger the number of semiconductor light-emitting elements arranged in an array is, the significantly lower the yield of semiconductor light-emitting devices is.
Furthermore, the above conventional semiconductor light-emitting device requires a pad electrode on a light extraction surface of each of the semiconductor light-emitting elements. Therefore, the larger the number of the semiconductor light-emitting elements arranged in an array is, the significantly smaller the area of effective light extraction surface becomes. As a result, there found a problem that sufficient light emission efficiency cannot be obtained.